Heretofore, plastic foams such as polyurethane foam which is used as a sealing material, caulking material or the like have been rendered waterproof.
As the method for rendering the plastic foam waterproof there have been known the following methods:
(1) To reduce the cell diameter and hence increase the surface area of the foam with the same density, providing a lower gas permeability; PA1 (2) To incorporate paraffin, coal tar, asphalt, polybutene, dialkyl phthalate, petroleum resin obtained by the polymerization of a C.sub.4-9 fraction in cracked naphtha, etc. in the components to be blended (see JP-A-55-71777 (The term "JP-A" as used herein means an "unexamined published Japanese patent application")); PA1 (3) To use a hydrophobic polyol and use a specific organosilicon compound as a foam stabilizer, rendering the foam water-repellent and hence enhancing the waterproofness of the foam (see JP-A-3-68677); and PA1 (4) To post-treat the foam with a commercial water repellent (e.g., Octex EM, available from Hodogaya Chemical Co., Ltd.), silicone wax, perfluoroalkyl compound or the like.
However, the foregoing methods (1) to (4) have the following disadvantages:
In accordance with the method (1), closed cells can be easily formed, giving a larger compression set. Therefore, when used as a sealing material or caulking material under pressure over a prolonged period of time, such a foam loses elasticity and remains distorted. When some trouble produces a gap between the foam and the adherend, the distorted foam cannot recover to fill up the gap, causing water leakage. Accordingly, such a foam cannot be used over a prolonged period of time and thus exhibits a poor durability.
Among the additives to be used in the method (2), paraffin, coal tar or asphalt provides the various components with a poor compatibility, giving ununiform cell shape, size and distribution. Further, since the various components, including plasticizer, exhibit a poor compatibility, they can bleed from and stain the surface of the foam in a relatively short period of time, making it impossible to maintain a desired waterproofing effect. Further, when a polybutene or petroleum resin is added, the resulting foam shows no change in physical properties but has a larger cell diameter that provides a higher gas permeability, impairing the effect that would be developed by the addition of the hydrophobic material.
In accordance with the method (3) which comprises the use of a specific polyol and foam stabilizer, the resulting foam becomes heterogeneous and thus exhibits dispersed gas permeability and water-sealing properties, making it impossible to exert a sufficient waterproofing effect. Further, such a foam is poor in physical properties such as compression set at a temperature as high as 80.degree. C. When a hydrophobic filler such as hydrocarbon compound is added to such a hydrophobic polyol, the shape and other properties of the resulting cells become uniform because the polyol is well compatible with the additives. However, it is difficult to reduce the gas permeability of the foam due to the effect of a large amount of additives, making it impossible to obtain sufficient water-sealing properties. In addition, the compression set of the foam is further worsened, causing a strain rise after a prolonged compression.
The method (4) is effected as a step following the preparation of the foam and thus normally requires complicated heating or pressing that reduces the productivity.